This invention relates to an image processing method to be applied to an image formed by scanning a color photograph to make it a set of color image signals based on a silver halide photosensitive material and an image processing apparatus which is suitable to the use in the practice of said processing method, and in particular, to an image processing method and apparatus requiring a small amount of calculation work and being capable of suppressing a granular noise signal contained in the color image signals and emphasizing the sharpness of the image, while preserving the shade in the periphery of the bridge of the nose and the shade around the eyes, without producing a noise looking like a color deviation (deviation of colors due to a registration error in a color print, hereinafter referred to as a color deviation simply) and a smooth blank makeup face, or without generating a noise looking as if fine powders are scattered.
It is put in practice that an image formed on a color photographic film is photoelectrically read by a CCD image sensor or the like, and transformed into a set of color image signals. Such color image signals, after being subjected to various kinds of image processing typically represented by negative-to-positive reversing, luminance adjustment, color balance adjustment, granular noise removal, and sharpness emphasis, are transmitted through a recording medium such as a CD-R, a floppy disk, or a memory card, or an Internet line, and are outputted as a hard-copy image by a silver halide photographic paper, an ink jet printer, a thermal printer, or the like, or displayed on a recording medium such as a CRT, a liquid crystal display, or a plasma display, to be appreciated.
Generally speaking, a color photographic image is formed by a congregation of dye-clouds of various sizes. Therefore, if the image is observed by enlarging it, it is found that mottled granular unevenness based on the dye-clouds are present in areas which should have a uniform tone originally. Caused by this, in a color image signal which is obtained by photoelectrically reading an image formed on a photographic film by a CCD image sensor or the like, a noise signal corresponding to the unevenness is contained. It has posed the problem that this noise signal became remarkably large accompanied especially by image processing for emphasizing sharpness, which degraded the quality of the image.
As regards a method of removing a noise contained in an image signal, a smoothing method and a method using a median filter are widely known (for example, ‘S. Inoue et al, “Learning practical image processing using C language (written in Japanese)”, p. 54, published by Ohm Co., Ltd.’).
In U.S. Pat. No. 4,812,903, it is proposed a technology for emphasizing sharpness while suppressing granularity through it that a set of color image signals are transformed into a luminance signal and color information signals, the luminance signal is further decomposed into a low-frequency component and a high-frequency component, non-linear processing is applied to said low-frequency component of the luminance signal, an emphasis processing is applied to said high-frequency component of the luminance signal, and an adjustment processing is separately applied to said color information signals; after that, the low-frequency component of the luminance signal, the high-frequency component of the luminance signal, and the color information signal, which have been processed, are combined to give a set of processed color image signal.
Further, in the publication of the unexamined Japanese patent application S63-26783, it is proposed a technology for obtaining a natural emphasis processing with a small change of color tone through it that a set of inputted color image signals are transformed into a luminance signal and color information signals, spatial general information is abstracted by applying an averaging filter processing to said luminance signal, spatial detailed information is calculated from the difference between said luminance signal and said spatial general information, and a specified transformation processing is applied to each of said spatial detailed information and said spatial general information; after that, the spatial detailed information, the spatial general information, and said color information signals, which have been already processed, are combined to give a set of processed color image signals.
In the publication of the unexamined Japanese patent application H9-22460, it is proposed a technology for emphasizing sharpness while suppressing granularity through it that an inputted image signal is decomposed into a low-frequency component, a medium-frequency component, and a high-frequency component, an emphasis processing is applied to said high-frequency component, while suppression processing being applied to said medium-frequency component, and the processed high-frequency component, the processed medium-frequency component, and the low-frequency component are combined to give a processed image signal. Further, also it is proposed in the same publication a technology for suppressing the roughening of color through abstracting a luminance component from the aforesaid high-frequency component and medium-frequency component in process of the above-mentioned processing, and practicing the above-mentioned emphasis suppression processing on the basis of said luminance component only and composition.
In the publication of the unexamined Japanese patent application 2000-215307, it is proposed a technology for suppressing the roughening resulting from granularity and enhancing sharpness through it that a set of inputted color image signals of R, G, and B are decomposed into a low-frequency component, a medium-frequency component, and a high-frequency component, it is obtained a correlation value with respect to corresponding pixels between at least one color set composed of two colors out of the above-mentioned three colors R, G, and B of said medium-frequency component and/or said high-frequency component, a granularity detecting processing using a specified morphology operation is applied to the above-mentioned R, G, and B signals, and an emphasis processing is applied to said high-frequency component, while a suppression processing is applied to said medium-frequency component on the basis of evaluation values composed of the above-mentioned correlation value and the result of the granularity detecting processing using the above-mentioned morphology operation; then, the above-mentioned high-frequency component after the emphasis processing, the above-mentioned medium-frequency component after the suppression processing, and the above-mentioned low-frequency component are combined to compose a signal. Further, also it is proposed in said publication a technology such that, prior to the above-mentioned emphasis processing to the high-frequency component, from the aforesaid medium-frequency component and the aforesaid high-frequency component, a luminance signal concerning said medium-frequency component and a luminance signal concerning said high-frequency component are abstracted respectively, and the aforesaid emphasis processing to the high-frequency component is an emphasis processing to the luminance signal concerning the aforesaid high-frequency component, while the aforesaid suppression processing to the medium-frequency component is a suppression processing to the luminance signal concerning the aforesaid medium-frequency component.
However, because the noise removal by means of a simple filter processing which is widely known is accompanied by the lowering of image sharpness, a satisfactory image cannot be obtained. In a technology disclosed in U.S. Pat. No. 4,812,903 or in a technology disclosed in the publication of the unexamined Japanese patent application S63-26783, although some effect can be recognized in the suppression of granular noise being made worse in a sharpness emphasizing process can be recognized, it is not sufficient, and it cannot be expected to suppress a granular noise for a signal before the processing.
The method disclosed in the publication of the unexamined Japanese patent application H9-22460 has an effect to eliminate granular unevenness looking mottled in an image by selectively suppressing a medium-frequency component in which a granular noise is mainly present, but because the information on the image structure which is present in the medium-frequency component is suppressed at the same time, for example, the shade in the periphery of the bridge of the nose and that around the eyes tend to be suppressed to give an impression of blurred image of the face. If the high-frequency component is remarkably emphasized for compensating this feeling of blur, the image becomes unsightly due to the generation of fine noises looking like color deviations in the flat areas such as the cheek.
According to the method proposed also in the publication of the unexamined Japanese patent application H9-22460 in which emphasis suppression processing and combining are practiced on the basis of the luminance component of medium and high frequencies only, the noise looking like a deviation is not generated, but a subtle variation in a reddish color of the skin etc. are also suppressed, which gives an impression of smooth blank makeup face.
Further, in any one of the examples of the embodiment described in the publication of the unexamined Japanese patent application H9-22460, if it is attempted to enhance the sharpness in fine areas like hairs, the high-frequency component is excessively emphasized, and it is generated a noise looking as if fine powders are scattered over the whole image area.
Further, there is another problem that, in order to separate a low-frequency component from an image signal as intended in the publication of the unexamined Japanese patent application H9-22460, it is necessary to apply a processing with a very large-sized low-pass filter to every channel of the color signal representing the original image (for example, each of the signal channels of B, G, and R), which makes the load of calculation for the image processing very heavy; this is also a very big problem.
The method proposed in the publication of the unexamined Japanese patent application 2000-215307 eases somewhat the above-mentioned defect by dynamically controlling the degree of the suppression of the medium-frequency component and the degree of the emphasis of the high-frequency component in accordance with the situation, but it cannot be said that it is sufficient, and it follows the method which has the point of problem in the image quality as it is. From the viewpoint of the amount of calculation, it is needless to say that this method has a heavier load than that proposed in the publication of the unexamined Japanese patent application H9-22460.
On the basis of the above-mentioned situation, it is an object of this invention to provide an image processing method and apparatus requiring a small amount of calculation work and being capable of suppressing a granularity noise signal contained in an color image signal and emphasizing the sharpness of the image at the same time, while preserving the shade in the periphery of the bridge of the nose and the shade around the eyes, without producing a noise looking like a color deviation and a smooth blank makeup face, or without producing a noise looking as if fine powders are scattered.